1. Field of the Invention
This invention relates to methods of oscillating molds for continuous casting at high frequencies and molds oscillated by such methods. More particularly, it relates to methods of oscillating at high frequencies molds that are used in the continuous casting of billets, blooms and slabs of metals and molds that are oscillated at high frequencies while such semi-finished products of metals are being continuously cast.
2. Description of the Prior Art
It has been known to provide a larger number of high-frequency ocillating means (hereinafter called oscillators) on the mold to oscillate the inner wall of the mold near the meniscus of liquid metal during the continuous casting operation, as, for example, is disclosed in Japanese Provisional Patent Publication No. 55742 of 1987.
FIG. 1 shows an example of a continuous caster mold 1 provided with oscillators 9a to 9l. The mold 1 has an inner lining of copper 4 on the inside of broad-face plates 2 and narrow-face plates 3. The inner lining 4 is oscillated by the oscillators 9a to 9l connected thereto. To prevent the seizure or sticking of liquid metal to the inner lining of the mold 1, it is necessary to continuously oscillate the entire surface of the inner lining 4 of the mold 1 near the meniscus at desired frequencies. To the oscillators 9a to 9l are connected a frequency generator 6, a power setter 7 and an amplifier 8 successively, as shown in FIG. 2. The frequency generator 6, power setter 7, amplifier 8 and oscillators 9a to 9l constitute a set of oscillating means 5. The oscillating means 5 sets the frequency and power with which the inner lining 4 is oscillated.
Oscillators having the same oscillating characteristics are commonly employed. Furthermore, a set of oscillators used in the conventional oscillating methods accomplish oscillation with the same frequency. Therefore, the high-frequency waves transmitted from the oscillators A and B interfere with each other at the interface between the inner lining 4 and liquid metal or the solidified shell M, as shown in FIG. 3. If the high-frequency waves from the two sources are of the same phase, the amplitude of frequency will be doubled to cause violent oscillation at point P.sub.1 on the inner lining that is at distance AP.sub.1 =BP.sub.1. On the other hand, the amplitude at point P.sub.2 where AP.sub.2 -BP.sub.2 =.lambda./2 (where .lambda.=wavelength of high-frequency wave) will become very small, with the high-frequency waves from the oscillators A and B offsetting each other. The result is the occurrence of seizure or sticking.
Graph (a) of FIG. 4 shows how offsetting occurs at point P.sub.1. Dotted line shows the high-frequency wave from the oscillator A, chain line shows that from the oscillator B, and solid line indicates the composite wave obtained by combining the two, all at point P.sub.1. Similarly, graph (b) of FIG. 4 shows the offsetting condition at point P.sub.2.
In the method of oscillating a mold provided with a plurality of oscillators according to Japanese Provisional Patent Publication No. 57742 of 1987, the difference between the high-frequency waves generated by adjoining oscillators for the oscillation of the inner lining is kept within the limit at which beat is produced.
The frequency of the waves generated by one oscillator can be varied by controlling the frequency setter. But if the frequency of an oscillator (of the electrostrictive or magnetostrictive type) that produces the maximum amplitude at frequency f.sub.0 is lowered under (f.sub.0 -1) KHz or raised above (f.sub.0 +1) KHz, the amplitude will become very small as shown in FIG. 5. The method of Japanese Provisional Patent Publication No. 57742 of 1987 greatly varies the frequencies of the individual oscillators. But if the oscillators have the same oscillating characteristic, the amplitude of high-frequency waves produced by some oscillators is then decreased so greatly, as mentioned previously, that the inner lining is not oscillated with large enough amplitudes. If, on the other hand, oscillators of different types having different oscillating characteristics are used, difficult problems will arise in the control and management thereof.
The separately excited oscillation generator that drives the oscillator is an open-loop control system in which power varies with variations in load (or variations in impedance). Therefore, it has been difficult to keep constant the amplitude of oscillation. With light loads, the amplitude of oscillation varies greatly as frequency varies, as indicated by dotted lines in FIG. 6. Such oscillations are commonly controlled by such automatic frequency tracking constant amplitude control circuits as are shown in FIGS. 7 and 8.
This type of automatic frequency tracking constant amplitude control circuits detect the amplitude of oscillation by the use of the following equations expressing the relationships among the voltage E at the oscillator terminal, current I, control impedance Z.sub.d, speed of the mechanical terminal v and coefficient of power A: EQU E=Z.sub.d I+Av=(Z.sub.d +Z.sub.m)I (1) EQU Z.sub.m I=Av (2)
As shown above, the impedance of the oscillator is expressed as the sum of the control impedance Z.sub.d that is independent of oscillation and the control impedance Z.sub.m that depends on oscillation. Therefore, the voltage proportional to oscillation is obtained by subtracting the voltage drop due to the control impedance Z.sub.d from the voltage at the terminal of the oscillator. The bridge circuit of an oscillator and impedances Z.sub.1 to Z.sub.3 shown in FIG. 7 is an example of concrete sensing methods commonly employed for the detection of the output voltage E.sub.2 that is proportional to Z.sub.m I.
Automatic frequency tracking is accomplished by means of a closed circuit formed by a high-frequency oscillator amplifier circuit 14 (transfer function in the amplifier circuit: .mu.) and the oscillation sensing circuit shown in FIG. 7, which constitutes a feedback circuit 17 (transfer coefficient in the beedback circuit: .beta.). The oscillating condition in this circuit is as follows: EQU .mu..beta.=1 (3)
Then, the frequency to satisfy the following equation is automatically chosen: EQU &lt;.mu.+&lt;.beta.=2n.pi.(n: integer) (4)
The constant amplitude control circuit shown in FIG. 8 compares, in a voltage comparison control circuit 13, an output signal preliminarily set by the amplitude setter 12 with a signal produced by amplifying the voltage E.sub.2 from the oscillation sensing circuit by a voltage input amplifier 18. Then, the voltage comparison control circuit 13 inputs a control signal into the oscillator amplifier circuit 14 consisting of a resonant phase circuit 15 and an output matching inverter 16 to control the output to the oscillator so that constant amplitude is maintained at all times.
With the automatic frequency tracking constant amplitude control, however, it is impossible to arbitrarily vary the frequency of oscillations produced by adjoining oscillators so that the amplitude of oscillations in the area to be oscillated is effectively flattened. This can results in uneven amplitude that leads to seizure and sticking.
Exposed to rapidly flowing cooking water and oscillated at high frequencies, the water-cooled oscillated surface of the inner lining is susceptible to cracking and erosion. Japanese Provisional Patent Publications Nos. 197351 and 197348 of 1984 disclose methods of preventing such cracking and erosion by covering the weak spot in the water-cooled oscillated surface with a sheet of cushioning material or alloyed metal. Although effective in decreasing the occurrence of cracking and erosion, those methods are not without problems. In the course of long-time service, for example, water may penetrate into a space between the attached covering material and the water-cooled oscillated surface, causing erosion. The covering material coming off may clog up the cooling water passage. A more important problem is that the covered portion of the inner lining is not cooled adequately. Such being the case, development of a better oscillated mold capable of withstanding long-time service has been awaited.
Oscillators are usually cooled by water-cooling, air-purging or other means as overheating can result in their breakage. If any of the oscillators malfunctions, the composite oscillation applied to the mold will become different from the originally intended one, thereby impeding the smooth implementation of the continuous casting operation. Permitting no water cooling because of the insulation consideration necessitated by the applied voltage as high as, for example, 4000 V.sub.P-P, electrostrictive oscillators are cooled by air-purging etc. Because air-purging and other similar cooling methods are not so effective as water-cooling, operation of the electrostrictive oscillators should be watched carefully.
Monitoring of oscillators has been performed by measuring the voltage and current of the power supply servicing the oscillators. But it is difficult for this method to grasp the degree of deterioration in the electrostrictive elements in the oscillators because it does not perform direct measurement of oscillations. Therefore, oscillators often break unexpectedly, offering an obstacle to the continuous casting operation. Another conventional method of monitoring the operation of oscillators measures amplitude with an amplitude detector. But this method is costly because a large number of amplitude detectors and amplifiers must be provided to cover a large number of oscillators attached to the oscillated mold. Furthermore, this method has not been very reliable because amplitude detectors are apt to come off easily. "Handbook of Ultrasonic Technologies" (Nikkan Kogyo Shimbum) discloses, between pages 488 and 490, various types of pickup sensors that can be used for measuring microamplitudes of oscillating solids. But they are costly and difficult to attach. Their sensors are apt to come off during the long-time service. They need much larger installation space than the high-frequency oscillated mold of continuous casters can afford.